Abstract
Abstract. A rapid development of a severe air pollution event in Beijing, China, at the end of November 2015 was investigated with unprecedented observations collected during the field campaign of the Study of Urban Rainfall and Fog/Haze (SURF-15). Different from previous statistical analyses of air pollution events and their correlations with meteorological environmental conditions in the area, the role of turbulent mixing in the pollutant transfer was investigated in detail. The analyses indicate that the major pollution source associated with high particulate matter of diameter 2.5 µm (PM2.5) was from south of Beijing. Before the day of the dramatic PM2.5 increase, the nighttime downslope flow from the mountains to the west and north of Beijing reduced the surface PM2.5 concentration northwest of Beijing. The nighttime surface stable boundary layer (SBL) not only kept the relatively less-polluted air near the surface, it also shielded the rough surface from the pollutant transfer by southwesterly winds above the SBL, leading to the fast transport of pollutants over the Beijing area at night. As the daytime convective turbulent mixing developed in the morning, turbulent mixing transported the elevated polluted air downward even though the weak surface wind was from northeast, leading to the dramatic increase of the surface PM2.5 concentration in the urban area. As a result of both turbulent mixing and advection processes with possible aerosol growth from secondary aerosol formation under the low-wind and high-humidity conditions, the PM2.5 concentration reached over 700 µg m−3 in the Beijing area by the end of the day. Contributions of the two transporting processes to the PM2.5 oscillations prior to this dramatic event were also analyzed. The study demonstrates the important role of large-eddy convective turbulent mixing in vertical transfer of pollutants and the role of the SBL in not only decoupling vertical transport of trace gases and aerosols but also in accelerating horizontal transfer of pollutants above.
Highlights
Rapid urbanization in China associated with its fast growing economy has led to heavy air pollution, which has drawn international attention
The number of “blue sky” days measured by the air quality index (AQI) increased in 2015 from its 2014 number according to the state bulletin issued by the Chinese Ministry of Environmental Protection (MEP), air pollution is still a serious issue especially in the region of Beijing–Tianjin–Hebei (BTH) and its surrounding area including provinces of Shanxi, Shandong, Henan, and Inner Mongolia
Similar to previous pollution studies, we found that the slow movement of the regional pressure system and the influence of the mountains northwest of Beijing provided favorable conditions to keep polluted air in the Beijing area
Summary
Rapid urbanization in China associated with its fast growing economy has led to heavy air pollution, which has drawn international attention. We concentrate on a severe air pollution event through detailed analyses of aerosol transporting mechanism with both turbulence and aerosol observations from an intensified field campaign in the Beijing area. We use 5 min measurements from the dense network of automatic weather stations (AWSs) operated by Beijing Meteorological Service (BMS) These data include wind speed and direction (EL15-2C) at 10 m, temperature and humidity (HMP155, Chinese Huayun Company) at 1.5 m, and pressure (PTB210, Chinese Huayun Company) at 1.5 m observed at Changping (CHP), Chaoyang (CHA), Mentougou (MEN), Daxing (DAX), Tongzhou (TON), and HAI. At 01:00 LST, 30 November, the surface PM2.5 concentration was high south of Beijing (Fig. 5) This polluted air was visibly confined east of the Taihang Mountains from the MODIS images on 29 and 30 November (Fig. 6), which is often observed in winter (e.g., Xu et al, 2005; Zhao et al, 2013; Zhong et al, 2017).
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